A variety of semiconductor processing operations are commonly performed in diffusion furnaces, which in a modern semiconductor wafer fabrication facility frequently include two "stacks" of diffusion furnaces placed side by side. Each stack typically includes four horizontal quartz "diffusion tubes", each approximately eight feet long, positioned each above the other in a "diffusion furnace". The two stacks are positioned back to back, each being accessible from an opposite side. At one end of each stack is a "source cabinet" in which connections to controlled sources of various reactant gases can be made to the "pigtail" end of each diffusion tube. The opposite "mouth" end of each diffusion tube extends into a "scavenge box" into which used reactant gases are exhausted and conducted to a "scrubber" that performs the function of burning off certain components of the exhausted gases. A "load station" for each diffusion tube is connected to the loaded end of each diffusion furnace.
Those skilled in the art will realize that the foregoing arrangement of back to back stacks is necessary to minimize the amount of floor space required because it is known that an ultra-pure environment must be maintained in a modern wafer fabrication facility to avoid, to the greatest extent possible, the existence of particulates, even those in the range from 0.5 microns to 4 or 5 microns in diameter, in the ambient air. This is because it is well known that particles of this size can cause defects in the integrated circuits being manufactured in the wafers. The resulting decrease in wafer yield (and hence the increase in fabrication cost per integrated circuit) increases with the density of such particulates in the wafer fabrication environment. As state of the art of integrated circuits proceeds toward minimum line widths, and line spacings are reduced toward one micron, the minimum size of a typical particle that will cause a catastrophic defect in an integrated circuit becomes smaller and smaller. Tremendous amounts of capital have been invested by the semiconductor industry over the past decade or so to improve purity of the air and environment which is required for high yield wafer processing. Floor space in such a modern wafer fabrication facility is extremely expensive.
The various wafer processing operations mentioned above typically include semiconductor diffusion operations at high temperatures of over 1,000 degrees C., and also somewhat lower temperature processes, including thermal oxidation and LPCVD (low pressure chemical vapor deposition) processes such as deposition of silicon nitride or polycrystalline silicon on semiconductor wafers.
In order to perform the foregoing processing operations, it is necessary to load quartz diffusion boats, each holding typically 50 to 75 partially processed semiconductor wafers, into the open end of the quartz diffusion tube of a diffusion furnace. Often this has been accomplished using "paddles" which are silicon carbide platforms that are cantilevered; the wafers are loaded into a "hot zone" in the diffusion furnace, whereat the temperature of the wafers is elevated and stabilized at the desired level for the desired oxidation, diffusion, or a chemical vapor deposition process.
Problems
There are several problems with the related art. First, there is never a constant concentration of deposition chemicals in prior existing systems. This problem exists because the deposition chemicals are only entering the quartz tube from one location, and that being near the cool end of the tube. This problem should always exist because the LPCVD system works under low pressure. Therefore, the wafers closest to the vapor introduction will receive more than enough deposition but the wafers further away will have far less deposition because there is less unreacted vapor concentration remaining.
Second, the temperature in the typical LPCVD quartz tube is not as uniform as one might expect. The current tubes will hold the wafers in the hottest location in the oven, being away from the entrance of the oven. However, the one end of the tube is still cooler than the end furthest in the oven. The cooler end sticks out of the oven, the heat in the tube wall is drawn out to the cooler tube portion that is outside of the oven. The gases will absorb heat from expansion of the gas at the cooler end. The wafers at either end of the wafer stacks are not as hot since there is only one side of the wafer that is receiving heat from adjacent wafers that are radiating heat. As a result, the cooler wafers will have less material deposited thereon than the hotter wafers. Which can result in uneven deposits on the wafers, leading to either rejects, or at the least, unexceptable deposition process variations.
Considering the increased reduction in the size of die, and/or the increased amount of circuitry placed upon die, it is crucial to have finer controls over LPCVD process than were required in the recent past. Without these tighter controls on processes, the extremely small sizes in circuitry will not be able to be uniformly mass produced, or to meet the customers required tight quality control specifications, and to meet the manufactures need for low product defects.
It is noted that the above described problems, as well as other problems, are solved through the subject invention and will become more apparent, to one skilled in the art, from the detailed description of the subject invention.